Mechanism of Paper Surface Roughening (II)

Abstract

Release of internal stress has been referred to as one of the causes for surface roughening of paper. However, occasionally drying-induced stress is not corresponding to roughening. The authors assumed that roughening is not caused by release of in-plane internal stress built in drying, but of transverse internal stress built in transverse processes such as calendering and wet-pressing. The analytical procedure of Kubat for determining in-plane internal stress was attempted to apply to the transverse compressive mode to determine the transverse internal stress. A straight-line portion was found at higher initial stresses in the plots of relaxation speed per unit logarithmic time versus initial stress (Kubat's plot). The slope and the offset of this straight-line portion were varied with the forming process. The initial stress at a relaxation speed of 25N/log (s) was defined as an index of transverse internal stress. Consequently, the greater the calendering, wet-pressing and beating, the higher the transverse internal stress (the slower the relaxation). But, the amount of loaded calcium carbonate did not affect it. The transverse internal stress had influence on surface roughening of paper, but its effect was observed to be different between the papermaking processes. Besides, the transverse internal stress was likely to be associated with apparent sheet density depending on the fiber source and the way of sheet making. The authors considered that the degree of fiber collapse and viscoelasticity are closely related to the transverse internal stress.